I've used ski goggles with clear lenses. They're hard to find, but Scott makes them. They don't fog up, but I wear them over glasses, and my glasses fog up. I can ride OK without my glasses on, so that's what I do when it's really bitter cold (below zero). I am going to mess around with the vents on the goggles this winter to see if I can improve matters.

One important thing is to arrange your muffler so your breath doesn't enter your goggles. I'm thinking of getting a neoprene balaklava.